1. Field of the Invention
The present invention relates to a suction recovery method for maintaining and recovering an ink ejecting performance of an ink jet printing apparatus and an ink jet printing apparatus in which suction recovery is performed by the suction recovery method.
2. Description of the Related Art
In an ink jet printing apparatus in which a minute ejection opening ejects ink droplets for printing, volatile ink components evaporate from the ejection opening of the ink jet head to cause ink to have an increased viscosity, increased ink dye concentration, or fixed ink for example. When an ink flow path of an ink jet head is left for a long time and bubbles are caused in the ink flow path, ink may be prevented from being supplied correctly to cause a significant inconvenience in the printing operation.
In order to avoid the inconvenience as described above, a suction recovery method has been used by which an ejection opening face of an ink jet head is capped and a negative pressure generation means (e.g., tube pump) is used to reduce the pressure in the cap to suck ink from the ejection opening. This method forcedly sucks, from the ejection opening, ink, bubbles, dusts or the like left in the ink flow path that is/are not suitable for a printing operation so that the quality of ink in the ink jet head and a correct printing can be maintained. Then, ink is discharged and new ink is filled in the ink flow path from an ink tank (ink storage section) via a filter.
This suction recovery method may use a tube pump. The suction recovery method using the tube pump is performed by handling a tube by the rotation of a roller. The rotation of a roller causes a negative pressure in the tube. By using such a principle, the tube pump performs the suction recovery operation based on suction conditions (e.g., suction pressure) that can be determined by controlling a motor for rotating the roller during a pump operation (by mainly setting the number of rotations and a rotating speed). However, the suction recovery method by the tube pump has a mechanism to increase the suction pressure while sucking ink, thus failing to set the suction amount and the suction pressure independently.
This suction recovery using the tube pump adjusts the suction amount by setting the number of rotations of the pump driving motor. The time during which the suction is performed is calculated by dividing the number of rotations by the rotating speed. Thus, when the suction recovery is performed by the tube pump, the pump driving motor is set to have an appropriate number of rotations to fill new ink to the ink flow path while sucking ink. When ink is filled to the ink flow path, discharged ink sucked to the cap is discharged from the cap to outside. At the same time, the negative pressure in the cap disappears.
However, when it is desired to secure a predetermined suction amount to set operational conditions, suction by a high-speed rotation as shown by IH of FIG. 9 causes a steep ascending curve IH of the suction pressure. This may cause an excessive increase of the suction pressure to cause an increase of the ink flow rate. Since an ink tank has an ink supply capability per a unit time having an upper limit, the ink flow rate exceeding the upper limit may cause a bubble to enter the ink flow path of the ink jet head when ink is supplied.
This phenomenon may be caused when a head 50 having a plurality of ink flow paths for inks of multiple colors as shown in FIG. 10 has significant differences in the flow path length to the ejection opening among the respective ink flow paths for the colors. Flow resistance is generally in proportion to the flow path length. Thus, when the respective nozzles have an identical diameter of an ejection opening, pressure loss as flow resistance will increase with an increase of the flow path length, thus suppressing ink from being sucked. A short flow path on the other hand allows ink to be sucked easily. Specifically, as shown in FIG. 11, even after bubbles are sucked by a short flow path (negative pressure curve: ILmin), a negative pressure is required in the cap to continuously suck ink until tLmax bubbles are entirely sucked from a long flow path (negative pressure curve: ILmax). In this status, a short flow path has smaller resistance than that of a long flow path and thus a negative pressure curve steeply rises. Thus, the short flow path causes a suction pressure (PLmin) to increase to a level equal to or higher than a suction pressure PT, thus causing an ink supply failure that causes bubbles to be sucked into the flow path.
A method for coping with the case is disclosed in Japanese Patent Laid-Open No. 2001-063102 for example. This method is a method to continuously suck ink to provide a pressure close to a target negative pressure maintained until bubbles are removed, after a pressure reach to negative pressure Pc at which an ink supply failure is not caused. Then the tube pump repeats driving and stopping a plurality of times so that ink suction is continued. FIG. 11 shows then negative pressure curve IPc.
Suction recovery in an ink jet head requires different target negative pressures depending on the size of an ejection opening diameter. A nozzle having a small ejection opening diameter (hereinafter referred to as small nozzle) requires a higher target negative pressure required for the suction recovery than in the case of a nozzle having a large ejection opening diameter (hereinafter referred to as large nozzle). The reason is that a meniscus force in a small nozzle is higher than a meniscus force in a large nozzle and thus ink suction from a nozzle must be performed by such a negative pressure that exceeds the meniscus force of the nozzle.
One of currently-developed ink jet heads has a design in which, in order to realize a printing operation with high-definition and a high speed, a single ink flow path 70 includes large and small nozzles having different ink ejection opening diameters as shown in FIG. 12. In the case of the suction recovery in an ink jet head as described above, the nozzle section 72 having a small ejection opening diameter has a higher meniscus force than that of a nozzle section 71 having a large ejection opening diameter. Thus, the ink suction in this case requires a target negative pressure (PN) that is further higher than a target negative pressure (PF) for sucking ink from the large nozzle. In order to suck ink from these nozzles having different ejection opening diameters by a single capping operation, an approach may be considered in which a suction pressure is steeply increased as shown by a negative pressure curve IH of FIG. 9 to suck ink from the small nozzle. However, this approach causes the large nozzle to have an ink flow rate exceeding an ink supply capability, causing a risk of an ink supply failure in which bubbles may be invited in the ink flow path. In the case of another approach in which ink is sucked without steeply increasing the suction pressure shown in a negative pressure curve IL, ink suction is continued until a target negative pressure required for filling ink in the small nozzle even after ink is filled in the ink flow path leading toward the large nozzle through which ink is easily sucked, thus increasing waste of ink.
With regards to ink suction from nozzles having different ejection opening diameters by a single capping as described above, a method is disclosed by Japanese Patent Laid-Open No. 2005-059554. This method firstly performs the ink suction with a small suction pressure and a negative pressure suitable for an ink flow path and a large nozzle to fill ink in the large nozzle and the ink flow path to subsequently suck ink from the remaining small nozzle with a high suction pressure instantaneously. Specifically, this method uses two different suction pressures. The negative pressure curve by the suction recovery as described above is shown by IFN in FIG. 9.
However, in the case of a head 90 as shown in FIG. 13 designed for providing further higher definition and speed, the respective nozzles have different ink flow path lengths since the number of nozzles is significantly increased. In addition, ink flow paths for the respective colors have nozzle ejection openings having different diameters.
Thus, in the ink jet head as described above having a structure in which ink flow paths have different lengths and ejection openings have different diameters, each ink flow path requires a different negative pressure required for suction recovery. Thus, it is more difficult for such an ink jet head to provide suction recovery of many nozzle groups by a single capping operation as in the conventional structure. An increased difference in the cross sectional area between nozzles shown in FIG. 12 tends to cause a deteriorated balance between the entire suction amount and suction amount required by the respective ink flow paths. Specifically, such ink suction conditions must be set that apply a high suction pressure to a small nozzle section 92 in FIG. 13 to realize ink suction and that also provide, with regards to ink suction from a large nozzle section 91 from which ink is easily sucked, an ink flow rate that does not exceed an ink supply capability per a unit time. However, it has been difficult to provide such ink suction conditions satisfying both of the former and latter factors.
In order to prevent such a deteriorated suction balance, ink suction is desirably performed so as to minimize pressure loss in nozzle sections by reducing the ink flow rate during ink suction. An increased ink flow rate during an ink suction causes a proportional increase of pressure loss in the respective nozzle sections functioning as resistance. This causes an increased pressure loss in the respective ink flow paths and nozzles to cause further deteriorated suction balance among the respective ink flow paths.
Thus, an ink suction operation is desirably performed with a low-speed rotation so that an ink flow rate is prevented from steeply increasing and the cap has therein a low target negative pressure. As shown in the negative pressure curve IL of FIG. 14, the low-speed rotation allows the cap interior to have a saturated pressure Psat. Thus, this is effective for an ink suction operation that fills, without causing an increased suction pressure, ink to an ink flow path having a high capacity and a large nozzle.
When ink flow paths for the respective colors include therein bubbles, a balance in the suction power among the nozzles for the respective colors further deteriorates. Thus, in order to uniformly transmit a high suction pressure to all small nozzles for the respective colors, it is required, from scratch, for all ink flow paths to be securely filled with ink without bubbles or the like.
However, when an ink flow path including therein bubbles is subjected to a normal suction recovery with an excessively-low target negative pressure, there may be a case where an expanded bubble remains in an ink jet head as shown in FIG. 15 without being moved. Such a bubble left in the ink jet head after ink suction prevents the subsequent ink suction from stably filling ink in the ink flow path. Thus, it is required to securely suck ink prior to the ink filling operation. In order to suck a bubble in ink in an ink flow path, the ink flow path must be applied with a negative pressure so as to provide a suction power exceeding a meniscus force caused at the fluid level. An ink tank and an ink flow path in which ink flows have therebetween a filter. A filter is a member that is designed to trap foreign matter (e.g., dust) included in ink flowing to an ejection opening of a head to prevent the foreign matter from flowing into the ejection opening. Thus, a clearance in the filter is as small as a nozzle ejection opening. A smaller area of a fluid surface causes a higher ink meniscus force, thus causing a higher meniscus force in the filter. Thus, in order to suck a bubble from the filter, a suction negative pressure at some high level is required. In the case of a printing operation having a high ejecting duty requiring a large amount of ink supply in particular, a bubble left in an ink flow path or a filter section may cause a printing failure.
When an ink flow path is subjected to suction recovery with a high negative pressure on the other hand, this causes a proportional increase of an ink flow rate to cause an increased ink flow volume. Since an ink tank has an ink supply capability per a unit time having an upper limit, an ink flow volume exceeding the ink supply capability causes bubbles to be sucked into the ink flow path during ink supply.
Another method for bubbles left in a filter section is disclosed in Japanese Patent Laid-Open No. 2001-063102. This method continuously sucks ink with a target negative pressure that is sufficient for separating such bubbles from a filter section. However, this method suggested by Japanese Patent Laid-Open No. 2001-063102, which uses a control by which sudden stop and sudden rotation of a pump are repeated so as to maintain a negative pressure close to the target negative pressure, causes a significant burden to a driving system including the motor. Thus, a suction recovery unit must be designed to endure such a burden. This causes the suction recovery unit to have a large size, resulting in a higher cost for the manufacture.